There is known a four-wheel drive vehicle (4WD vehicle) including an engine, a transmission, and a transfer case provided with a 2-4 switching mechanism for switch-over between two-wheel drive (2WD) and four-wheel drive (4WD), and wherein a free wheel hub mechanism for making each of the driven-side wheels free by an intake pipe negative pressure in a two-wheel drive condition of the transfer case is disposed between each of the driven-side wheels and a driven-side axle. In a two-wheel drive condition, the free wheel hub mechanism disengages a driven-side wheel from the driven-side axle and makes it free to prevent a wasteful traveling resistance based on rotation of the driven-side wheel; while in a four-wheel drive condition, the free wheel hub mechanism causes the driven-side wheel and the driven-side axle to be locked with each other, thereby allowing the driving force from the engine to be transmitted to the driven-side wheel.
More particularly, as shown in FIG. 13, in a four-wheel drive vehicle 702 there are mounted an engine 704, a transmission 706, and a transfer case 712 having a transfer lever 708 and a 2-4 switching mechanism 710. A rear drive shaft 714 as a driving-side drive shaft and a front drive shaft 716 as a driven-side drive shaft are connected to the transfer case 712.
The rear drive shaft 714 connects to a rear differential 718 as a driving-side differential. A rear axle 722 as a driving-side axle with rear wheels 720 as driving-side wheels connected thereto is connected through the rear differential 718.
The front drive shaft 716 connects to a front differential 724 as a driven-side differential. A front axle 728 as a driven-side axle with front wheels 726 as driven-side wheels connected thereto is connected through the front differential 724. Between right and left front axles 728 and right and left front wheels 726 are provided right and left free wheel hub mechanisms 730 respectively.
As shown in FIG. 14, each free wheel hub mechanism 730 is disposed between the associated front axle 728 and a wheel hub 732 of the associated front wheel 726.
According to the construction of the free wheel hub mechanism 730, a spindle or sleeve 734 and bearings 736 are disposed between the wheel hub 732 and the front axle 728. Within a hub housing 738 attached to the wheel hub 732 are formed a two-wheel drive-side negative pressure chamber 742 and a four-wheel drive-side negative pressure chamber 744 separated by means of a diaphragm 740. Further, within the hub housing 738 is mounted an outer gear 746, and a slide gear 748 connected to the diaphragm 740 and adapted to slide on the front axle 728. Two- and four-wheel drive-side negative pressure ports 750, 752 are formed in the spindle 734. The two-wheel drive-side negative pressure port 750 and the two-wheel drive-side negative pressure chamber 742 are in communication with each other through a two-wheel drive-side negative pressure path which is constituted by first, second and third negative pressure paths 754, 756, 758 connected in series. The four-wheel drive-side negative pressure port 752 and the four-wheel drive-side negative pressure chamber 744 are in communication with each other through a four-wheel drive-side negative pressure path which is constituted by first, second and third negative pressure paths 760, 762, 764 connected in series.
Further, for maintaining the negative pressure paths in the free wheel hub mechanism 730 in a hermetically sealed condition, a first sealing member 766 is provided which seals the two- and four-wheel drive-side negative pressure chambers 742, 744. Likewise, a second sealing member 768 is provided for sealing the two-wheel drive-side negative pressure chamber 742 against the atmosphere, and a third sealing member 770 is provided for sealing the four-wheel drive-side negative pressure chamber 744 against the atmosphere.
In the free wheel hub mechanism 730, as shown in FIG. 14, an intake pipe negative pressure acts on the two-wheel drive-side negative pressure chamber 742 in a two-wheel drive condition, whereby the slide gear 748 is disengaged from the outer gear 746 to render the wheel hub 732 free with respect to the front axle 728. In contrast, in a four-wheel drive condition, as shown in FIG. 15, an intake pipe negative pressure is exerted on the four-wheel drive-side negative pressure chamber 744, whereby the slide gear 748 is brought into mesh with the outer gear 746 to lock the wheel hub 732 with respect to the front axle 728.
In an axle lock type two- to four-wheel drive, as shown in FIG. 16, an intake manifold 806 having an intake passage 804 formed therein is connected to an engine 802, a throttle valve 808 is mounted in the intake passage 804, two- and four-wheel drive-side negative pressure introducing passages 810, 812 are formed communication wise in the intake passage 804 on a downstream side of the throttle valve 808, a two-wheel drive-side diaphragm mechanism 814 is mounted in the two-wheel drive-side negative pressure introducing passage 810, an axle unlocking valve 816 is disposed at an intermediate position in the two-wheel drive-side negative pressure introducing passage 810, and a four-wheel drive-side diaphragm mechanism 818 is mounted in the passage 812. Further, an axle unlocking valve 820 is mounted at an intermediate position in the passage 812, an axle 822 is connected to the two- and four-wheel drive-side diaphragm mechanisms 814, 818 respectively, and an axle lock detecting switch 824 is attached to the axle 822 for detecting a four-wheel drive condition on the basis of operation of the axle 822.
The axle unlocking valve 816, axle locking valve 820 and axle lock detecting switch 824 are connected to a control means 826 which is a four-wheel drive controller, as shown in FIG. 17. Also connected to the control means 826 are a transfer switch 830, an indicator lamp 832 mounted within the passenger compartment of the vehicle, and a battery 834. The transfer switch 830, which is connected to a transfer lever 828, turns ON upon detecting a four-wheel drive shift position of the transfer lever 828.
According to this construction, as shown in FIG. 18, a shift operation of the transfer lever 828 to four-wheel drive causes a switch over from two- to four-wheel drive, and when the transfer switch 830 turns from OFF to ON (indicated at position "a"), the control means 826 detects the ON condition of the transfer switch 830, turns ON the axle locking valve 820 to introduce the intake pipe negative pressure and causes the four-wheel drive-side diaphragm mechanism 818 to operate, thereby locking the axle 822. Then, when the axle lock detecting switch 824 detects the locked position, it turns OFF the axle locking valve 820 to stop the operation of the valve and lights the indicator lamp 832 (indicated at position "b").
In contrast, upon switch-over from four- to two-wheel drive, the control means 826 detects an ON-to-OFF switch-over of the transfer switch 830 (indicated at position "c"), then turns ON the axle unlocking valve 816 to introduce the intake pipe negative pressure and operates the two-wheel drive-side diaphragm mechanism 814, allowing the axle 822 to operate. Further, after switch-over from ON to OFF of the axle lock detecting switch 824, the control means 826 turns OFF the indicator 832 (indicated at position "d") and also turns OFF the axle unlocking valve 816 to stop the operation of the valve (indicated at position "e").
Free wheel hub mechanisms of the type described above are described, for example, in Japanese Patent Laid-open Nos. 62-1641 and 2-117426.
According to the free wheel hub mechanism described in No. 62-1641, in an automotive two- and four-wheel drive switching system having a transfer clutch, there is provided a drive means which causes the transfer clutch to be engaged in interlock with engine stop, thereby allowing the switch-over from two- to four-wheel drive to be done automatically in interlock with engine stop so that even when the outside air temperature drops in a cold region for example, resulting in freezing of an actuator which actuates the transfer clutch into an engaged or released state, the vehicle can be ensured to be in four-wheel drive.
According to the free wheel hub mechanism described in the No. 2-117426, in a four-wheel drive vehicle having a free wheel switch for switch-over between engaged state and disengaged state of a free wheel mechanism, there is provided a lock means for locking the free wheel mechanism in its engaged state irrespective of the free wheel mechanism disengaging operation of the free wheel switch, thereby allowing the free wheel mechanism to be kept in its engaged state even upon turning OFF of the free wheel switch as long as the vehicle is running in a four-wheel drive condition. For example, the engaged operative condition of the free wheel mechanism is locked continually irrespective of an operational condition of the free wheel switch which is remotely operated. Even if a free wheel mechanism disengaging operation should be done erroneously by the free wheel switch during the vehicle running in the four-wheel drive condition, an actual disengaging operation is prevented.
In a conventional four-wheel drive vehicle with free wheels, however, if the negative pressure path in the free wheel hub mechanism is kept negative in pressure for a long time in order to maintain the locked state which is the four-wheel drive state or the free state which is the two-wheel drive state, the sealing member as a boundary member to the exterior (i.e. atmosphere) can fail and cause entry of muddy water or the like, with the result that the function and durability of the free wheel hub mechanism is deteriorated.
In the axle lock type two- to four-wheel drive as shown in FIGS. 16-18, since the detection of the axle position in the four-wheel drive condition is performed mechanically by the axle lock detecting switch, there is the inconvenience that a switch actuating component is needed. Additionally, since the axles rotate, it is difficult to use a mechanical switch.